Barbara Sibley was being a good mom—but she did not know she would have to walk into hell to prove it. Her eight-year old had left something at home and so she drove over to the Sandy Hook School to deliver it. But something was odd about the compound when she got there at around 9:30. “It’s an elementary school,” she recalls to TIME. “There are usually kids outside or something. It was very quiet.”

She ran into another mom at the front entrance, where visitors had to be buzzed in. That’s when it became clear something was amiss: the window next to the door had been shattered. “There was glass everywhere,” she says. “And that’s when we heard gunfire.”

The school, in the well-to-do town of Newtown in Connecticut (median income $111,506), had become the stage for the latest manifestation of American civic terror: the sudden appearance of a heavily armed assailant bent on mass murder. The setting was perverse: Newtown reported only one violent crime in 2010 and was considered one of the more attractive places in the country to raise a family. By the end of the onslaught, as many as 20 families would be bereft of their children—and the nation, led by a mournful and teary President Obama, would once again be confronted with the issue of guns in a free society.

The instant Sibley heard the gunfire, she ran for cover, sprinting across the parking lot to where dumpsters stood, hiding there while the carnage took place. Within the school, the gunman was stalking the halls. Rebecca Cox related to TIME what her mother, Sarah Cox, the school nurse, saw: “She heard a loud popping sound, so she got under her desk in her nurse’s office. She did see the gunman’s feet. He walked into her office. She saw his legs and the bottom of his feet and just held her breath. He didn’t know she was there and then he turned around and walked out.” Sibley would later be told that students could hear what was going on over the PA system as one two-section class seemed to take the brunt of the invasion. “All of the kids heard everything going on,” she says. “They heard the gunshots. They heard the cries for help. The whole school heard it.”

When sirens blared in the background, Sibley knew help was on the way. Firetrucks from the station down the block pulled up and, when the authorities gave a signal to Sibley and the other moms hiding out with her, the women ran, keeping low, to a shielding fire truck as an officer covered them with his weapon. About 10 or 15 minutes later, authorities started to release students one class at a time. She saw her child and went down to the fire house to await instructions and head counts. And that was when the horror became apparent—not with visible carnage but by absence. “It was apparent,” says Sibley, “that there was one particular class of students that was not coming out of the building.” She says, “Those parents were just devastated and hysterical, and it was awful.” In the street outside the firehouse, which was bedecked with wreaths and Christmas trees, one woman collapsed into another woman’s arms, screaming “Oh my God. Why did they take my baby?”

The killer gunned down 20 children—18 of whom perished in the school, two dying in the hospital; six adults were among the dead as well. The massacre’s toll makes it the second-deadliest shooting in U.S. history, surpassed only by the 2007 shooting at Virginia Tech University in which 33 people were killed. Officials said the assailant took his own life; the police reportedly did not fire a single shot. Adam Lanza, as the alleged gunman was identified in several press reports, was 20-years old, a year under the legal limit for gun ownership in Connecticut. The weapons he used, including the two 9 mm handguns reportedly found by his body, were allegedly registered to his mother Nancy. She was reported to be a teacher at Sandy Hook, and it may have been her class that was attacked. She too was listed among the dead—but it was not yet clear whether her body was found at the school or elsewhere.

The personal tale of the assailant is likely to take a familiar narrative arc. Already there is speculation of untreated mental illness or incapacity. In the age of Facebook and Twitter, his brother Ryan, 24, was apparently misidentified as the shooter until web denials came flooding through, not an hour after the wrong information emerged—and spread all over. Ryan lived in a second-floor apartment with three roommates in Hoboken, New Jersey. He had been taken in for questioning in the early afternoon — but by Friday evening had been cleared of any involvement in the massacre. Well into the night, reporters remained positioned with a line of cameras across the street, aimed directly at the five-story brick building

The police officers of Newtown were dispatched to another crime scene, away from the school, and speculation immediately focused on a house on Yogananda Street in Newtown where Adam Lanza reportedly lived with his mother. There were reports her body was found there.

In a speech delivered six hours after the tragedy, the visibly shaken President of the United States wiped away several tears, saying “As a country, we’ve been through this too many times, we’re going to have to come together and take meaningful action to prevent tragedies like this, regardless of the politics.” Later, a subdued demonstration by gun control advocates was held outside the White House. The day before the massacre, the National Rifle Association’s twitter feed had claimed that the organization’s Facebook page had notched a record 1.7 million “likes.”